Electric air cooling system for vehicles



Jail. 12, 1937. L E PRESE 2,067,837

ELECTRIC AIR COOLING SYSTEM FOR VEHICLES Original Filed July 14, 1934 Patented Jan. l2, 1937 UNITED sTATEs PATENT OFFICE ELECTRIC AIB COOLING SYSTEM FOB VEHICLES Joseph F. Frese, Baltimore, M11., assignor to Monitor Controller Company, Baltimore, Md., a corporation of Maryland Original application July Divided and this 1985, Serial N0. 21,053

Lt,l 1934, Serial No. application May 11,

9 Claims. (Cl. T12-239) compressor, a ian motor for circulating the air in the car past the refrigerating coil, and one or more motors for circulating air and water around the condensing coil.

In the operation of the system the fan motor for circulating air past the refrigerating coil ,isfirst started, and this motor may operate independently of the other motors to circulate the air. The motor or motors associated the condensing coil cannot operate except while the motor associated with the refrigerating coil is in operation because of inter-locking connections between the starter for the latter motor and a control circuit for the starter of the motor or motors associated with the condensing coil. 'This control circuit is completed through a relay which can be closed only when the fan motor associated with the refrigerating coil is in operation. When the relay is closed by the manual operation of a switch the motor or motors associated with the condensing coil can operate automatically when a thermostat in the control circuit closes, and providing that a low pressure switch associated with the refrigerating unit is also closed. The motor which operates the compressor cannot operate until and. unless the motors associated with the condensing coil are in operation. When the starter for the latter motors is closed it connects the con trol circuit to an extension thereof which passes through a high-preure switch on the refrigerating unit and thence to a starter for the motor which operates the compressor and the latter motor then operates. Should the high pressure switch open the control circuit, the compressor motor will stop, but the motors assoelated with the condensing coil will continue to operate, to cool the coil as rapidly as possible. Should the thermostat switch or the low pressure switch open, all of the motors except the ian motor associated with the refrigerating coil will stop.

The invention includes means, which formsv the subject of the present application, for cutting off the current to the control circuit ci the starter for the motor associated with the result from operating on low voltage.

refrigeraing coil when the voltage in the supply circuit falls below a predetermined value, in order to prevent injury to the motors which might The means for accomplishing this includes a low voltage relay and a relay for closing the control circuit to. said starter, so associated and connected that the jarring of the relays by the movement of the car will not interfere with maintaining the control circuit complete until the voltage drops to an exact predetermined value.

In the accompanying drawing wherein the cooling system is illustrated diagramatically, A indicates a generator operated from a car axle C, and B indicates a battery which is kept charged by the generator. The field strength of the generator is controlled by third-brush regulation and by a resistance I in series with the shunt field 2 and means, hereinafter described, ior varying the resistance in series with the field winding in accordance with the load on the generator. One brush of the generator is connected by conductor 3 yto the supply wire L and by conductor 4 to one terminal of the battery and the other terminal of the battery is connected by conductor 5 to the supply wire L'.

The other brush of the generator is connected by conductor S to the arm 1 of a contacter d adapted to connect conductor 6 to the supply wire L through conductor 8 which extends through seriescoil Si of a reverse current relay e to said supply wire. The shunt iield circuit is completed between the conductor 6 and the resistance l by conductor IIL The reverse current relay has an energizing coil li connected to the main brushes of the generator through conductors t, l2, i3, resistance Il. and conduc-l tors l5 and 3. When the voltage of the generator is above that of the battery the energizing coil causes the relay arm to close a circuit through the magnet 20 of contacter d as follows: from conductor I5 through conductor i6 to the contact il, thence to contact it, thence by conductor i9 to the magnet 2t, thence through conductor 2l, in shunt to resistance 2li, to contact 22 and contacter arm i te conductor f5, and thence to the generator. 'When the arm 'l' closes it connects the generator to the battery and the coil 9 acts cumulatively with the coil il to hold the reverse current relay closed as long as current is owing irom the generator to the battery. The contacter d also inserts resistance l in parallel with the energizing coil of the reverse current relay through Depression o! the start" conductor 23, thus reducing the current iowing in said coil after the reverse current relay has closed, this being made possible by reason of the iact that the series coil 9 acts cumulatively 4with the energizing coil to hold the relay closed as long as 'the voltage of the generator is above that oi' the battery. The closure of the com tactor d also opens a shunt around resistance 24, leaving this in series with the magnet 2li while the contactor is closed. When the gem erator slowsl down until Vits voltage is below that o! the battery, the reverse current relay opens, thus breaking the circuit to the magnet 2II of contactor d and the latter opens the circuit between vthe generator and the battery. The energizing coil of the reverse current relay is always connected to the generator, and therefore when the generator voltage rises above that ot the battery, the relay will close and the circuit from the generator to the battery will again becompleted. Y

The retrigerating unit comprises a compressor g from the outlet side of which a pipe 26 leads to a condensing coll 21 and from said coil a pipe 26 leads through an expansion valve 25 to a refrigerating coil 36 and a return pipe 3|' extends from the latter coil to the inlet side o! the compressor. An electric motor h operates the compressor; a motor i operates a fan to` blow air around the condensing coil; a motor 1 operates a pump Ic to spray water on said coils, and a fan mot'or m circulates the air in the car around the refrigerating coil.

In this system the air circulating motor m must be started before any of the other motors can be put in operation. 'I'his is accomplished by closing a push button switch marked "Start. By then closing a push button switch marked Run, motors i and 1, which circulate air and water around condensing coil 21, will be brought into operation, providing control circuits, which include a thermostat switch t, and a low pressure switch Il, are closed, and this will befollowed by operation of the compressor motor providing a high pressure switch l2 is also closed. vShould the high pressure switch open, the compressor motor will be stopped but the fan and pump motors i and a' will continue in operation in order to reduce' the pressure in the condensing coil as rapidly as possible.

'111e normally open start push button switch is `in a circuit which `extends from supply wire L' by way of conductor I4 through the coilot a relay n, thence through conductor J5 to "start" push button switch, thence by conductors 36 and 31 to the coil of relay o, thence by conductor Il to conductor 39 through push button switch marked Stop to supply wire L. switch will cause the magnets of relays n and o to be simultaneously energized and each will close a holding circuit for the other. 'Ihus relay n will close a holding circuit for the magnet of relay o as follows: from supply wire L' and conductor I4 through conductor 40 to the switch oi relay n, thence through conductor 4I and resistance 42 to conductor 31 and coil of relay o, and thence through conductors 38 and 35 to supply wire L. Relay o, through its switch arm 43, will close a holding circuit for the magnet of relay n as follows: from supply wire L through conductor I4 and coil of relay n, thence through resistance 44 and conductor 45 to contact 46, thence through switch arm 43 and conductors u and u to the supply wire L Thus. upon depression of the "start push butm ton switch. both of these relays will close, providing the voltage on the4 supply wires is above a predetermined value suilicient to operate the several motors without injury.

The relay o, in addition to establishing a circuit for the coil of relay n, controls a circuit for a starter P for the fan motor m through the engagement of the arm 43 with a contact. 41 connected to conductor 48 which leads to the magnets of said starter, as hereinafter explained. It is important that the contact arm oi.' relay o shall remain ilrrnly closed and not vibrate when the voltage drops close to the predetermined value at which the starter circuits should be opened to protect the motors. 1f this relay alone were depended upon to open the control circuit for the starter P, its magnet would have to be wound so as to release the arm 43 at the critical voltage and jarring of the train might cause its contact arm to vibrate and open the circuit above that critical voltage. Therefore the magnet of the relay o is wound so that it will attract the arm 4,2 strongly at the critical voltage while the relay 'n whose magnet circuit is controlled by relay o will be adjusted so as to open at the critical voltage. Hence the relay o will remain rmly closed until the low voltage relay n opens and breaks the circuit through the magnet of relay o.

The circuit for the starter P which is opened and closed-by the relay o extends from supply wire L through conductor 39, stop switch, conductor 36, arm 43 and contact 41 to conductor 46, thence through a resistance 46, which is nor--` mally shunted, to conductors and 5| thence through the coil of magnet 52 and by conductor 5I to supply wire L'. When the magnet 52 is energized contactor p completes a circuit through the shunt magnet 54 of auxiliary contacter p' from conductors 50 and 5i through the coil oi' said magnet, thence by conductor 55 to a contact 56 which is connected by the contactar arm 51 to conductor 53 and supply wire L'. Simultaneously with the closure of the circuit through the shunt magnet 54 of the auxiliary contactor, a circuit is closed through Aits series magnet 56 and through the circuits of the fan motor m. The series circuit extends from supply wire L' througli conductor 53 to contact 59, thence through the contactor arm 51 to conductor 6I, thence through the coil `of the series magnet 56 to conductor 6|, thence through a starting resistance 62 and thence through series field 62 to the armature of the motor, and thence by conductor 64 to the supply wire L. The shunt ileld circuit extends from the contactor arm 51 through conductor 65 to the shunt field coil 66 and thence by conductor 65 to supply wire L. The series and shunt coils of the starter will both attract the centrally pivoted contact arm 61 but the series coil will prevent the arm from moving until the fan motor m has accelerated and the counter-electromotive force has reduced the current in the series circuitl suiliciently to permit the series magnet to release the arm 61, when the arm will be moved to closed position by the shunt magnet. The arm 61 is connected to the conductor 66 and when the arm moves to closedl position it engages a contact 68 which is connected by conductor 69 to the armature circuit of the motor, and thereby the starting resistance and the series magnet 58 are shortcircuited. 'Ihe motor then operates at full speed. When the contact arm 61 moves to closed position, an insulated contact piece 10 opens a circuit around the resistance I9 and this is placed in series with the shunt coil 54 and with the coil of the magnet 52 to reduce the current flowing in said coils while the motor m is in operation.

The closure of the contacter p, through an insulated contact 1| en the contacter arm, shortcircuits a section I of the generator eld resistance i, through conductors 12 and 13, thus strengthening the current in the field coil 2, so that the current output of thegenerator will be increased to compensate for the current taken by the fan motor m.

The closure of the contact arm 61 of the contactor p' causes the insulated contact piece 10 thereon to bridge two stationary contacts 14 and 15 and thereby connect conductor 48 to conductor 16 which leads to a stationary contact 11 of a relay r which is open at this time. Since the conductor 48 is at this time connected to the supply wire L' through arm 43 of relay o and conductors 38 and 39 and the stop switch, the

contact 11 is connected to said supply wire by operation of the contacter p. The contacter p, in closing, connects the supply wire L', through conductor 53, arm 51 and contacts 59 and 18 with a conductor 19 which extends to the coil of magnet of relay r and thence by conductor 8| to a stationary contact 82 of a normally open push button switch, marked Run, adapted to complete a circuit through the coil 80.

Assuming the relays n and o and the contactors ef starter P to be closed and the air circulating motor m to be in operation, following the closure of the start push button, and the contact 11 of the relay r and contact 82 of the push button switch run te be connected to the supply wire L through the contaotors p and p of the starter P, respectively, as described, the next step is to close the push button switch run, which operation causes the relay r to close a control circuit for the starter S which latter controls the starting and stopping of the fan motor z' and pump motor i. The control cir-` cuit for the magnet 80 of the relayvfr extends from the supply wire L through conductor 39, stop push button, run push button, conductor 8|, coil 80, conductor 13 through interlock on contacter 'p and conductor 53 to supply Wire L. When the switch run is closed, this circuit is completed and the relay r closes. When the relay r closes, its Contact arm 8E closes a holding circuit for its magnet 80 in shunt to switch run through conductors 38 and 85 te arm 34, thence through resistance and conductor tl to coil 80.

When the relay r closes an insulated contact on its'arm completes the control circuit for the magnets of the starter S as follows: from the supply wire L through conductor 39 and the "stop switch and conductor 38 and arm 43 of relay o to conductor 48, thence through contacts 15, 10 and 15 of contacter p to conductor 16, thence through contacts 11, 90 and 9| of relay r to conductor 92, thence through low pressure switch 33 and conductor 94 through thermostat switch t te resistance 95, which is normally shunted, thence through conductor 96 to conductor 91 and coil v$18 of contractor s and thence through conductor 99 to supply wire L. Conductor 31 is also connected to, one terminal of the shunt magnet |00 of auxiliary contacter s', and the other terminal is connected by conductor lili to a contact |02 of contractor s. When the contacter arm |03 closes, it completes the circuit through the coil |00. Simultaneously the contacter s completes the circuit from its arm |03 through conductor |06 to the series cell |05 of contacter` s', thence through conductor |06 and starting resistance |01 to conductor |08 and series field ws of fan motor i, thence through the armature of the motor and by conductor H0 to the supply wire L. The series field and'armature of the pump motor 1' are connected in parallel with the series field and armature of the fan motor z' by conductor il@ which leads from conductor |08l to the series lleld ||2 of motor 7', thence through the amature of said motor and by conductor ||3 to the supply wire L. The shunt fields of these motors are also connected in parallel by conductor IM which extends from contacter arm |03 te shunt eld ||5 of motor j and thence to supply wire L, and a conductor H6, branching from conductor H4, extends through shunt eld ||1 of motor i and thence to the supply wire L.' When the magnets of the auxiliary contacter s are energized, vthe contacter arm ||8 is attracted by both magnets but the series magnet prevents the contacter arm from closing until the counter-electrometive force of the motors causes the series magnet |05 to' weaken, when it is over-powered by the shunt magnet and the contacter arm closes, short-circuiting the series magnet and the starting resistance through a conductor H9 which is connected from conductor |04 through arm ||8 to a contact connected to conductor HB. The motors i and y' then operate at full speed. An insulated contact |20 on the contacter arm ||8 -opens the shunt around the resistance and the latter is thereby placed in circuit with the coils of magnets 58 and |00, to reduce the current flowing therein.

From the foregoing, it will be seen that after the air circulating motor m has been started a circuit for the relay r is made up through the starter for said motor and when said circuit is completed by closing the lpush button run thee relay r completes a control circuit for the starter S through the low pressure switch and thermostat switch and if these are closed the starter S operates to start the fan and pump motors associated with the condenser coils.

The closure of the auxiliary contacter s' of the starter S completes the control circuit to a starter V which controls the circuit of the compressor motor h. `Thus, when the auxiliary centactor of the starter S closes, the insulated contact piece |20 on the arm H8 engages stationu ary contacts |2| and |22, the former connected to the control wire 94 leading via conductor 32' from relay r through the low pressure switch and the thermostat switch to the starter S, and

, the Contact 22 being connected by conductors |23 and |22 to conductor G25, thence through coil i255 of main contacter o, thence by conn ductor 21 to contacts t28 and 29 which are normally closed by the arm 30 of contacter 'thence by conductor |3I to supply wire When this extension |23-I24 of the control circuit is closed by the starter S, the contacter v is energized and its contact arm |32 completes the circuits of the compresser motor from supply wire L' through conductor |3| to contacter arm |32, thence through conductor H3 and starting resistance |34 to series coil |35, thence through the armature and conductor 36 to 'the supply wire L; The shunt eld circuit extends from contacter arm 32 and Vconductor i through conductor |31 and vshunt field S38 te the supply 75 wire L. Upon closure of the contactor v the compressor motor starts and a circuit is also completed by the contactor arm |32 from conductor through coil |39 of retarded relay w to a contact |40 which is connected by the contactor arm to contact |4l, which latter is connected by wire |3| to supply wire L. The core o! the relay moves upward, suitably retarded, as by a dash pot |42. A contact member |43 om the relay is connected by a conductor |44 to the) supply wire L', and as this contactv member moves upward it engages iirst a contact |45, which is connected by conductor |48 to coil |41 oi' auxiliary contactar o" and thence to conductor |25. When magnet |41 is energized, itsI arm |43 cuts out a part o! the starting resistance |34. As the contact member |43 of the retardedrelay continues to move upward it next engages a contact |43 which is connected by conductor |53 to the coil |5I of a relay :c and ythence by a conductor |52 to the supply wire L.

I'his relay hasa stationary contact |33 connected by conductor |54 to the wire 12 which is connected to the eld resistance o! the generator, and the arm` |55 oi' the relay is connected by conductor |55 to saidv resistance;

hence when the relay closes, the sections l* and l' ot the resistance will be short-circuited through the relay switch and the generator ileld will be strengthened by the cutting out of the section l* to increase the generator output in order to compensate for the load placed on the generator by the pump motor i and compressor motor h, the section l* having previously been cut out by the starter from the motor m. As the contact member o! the retarded relay w moves further upward, it engages a contact |53 which is connected by conductor |59 to the coil o! contacter vI and thence to the control wire |24-|2l. When this contacter closes, it cuts out the remainder of the starting resistance in the 'circuit of the motor h and establishes a holding circuit for the magnet lthrough resistance III, so that the contactor remains closed. The closure of this contactor opens the circuit through the coil of the magnet of contacter v and the arm of the latter falls to open position, opening the circuit through the coil of the retarded relay w. The contact member o! the latter relay then drops downward, interrupting the circuit through wire |59 to the magnet oi relay o. which magnet is now energized through its'holding circuit, and also interrupting the circuit through wire |50 to the cut-out relay which is now kept energized by current owing through contacter v2, conductor |33. and resistance |52 to conductor |50, and ilnally opening the circuit through contacter v'.

For testing purposes, a circuit |63, in which is a hand switch |54, is connected to conductors 32 and 34 in shunt to the low pressure switch 33 and thermostat switch t. By closing this switch when the relay r is closed, the motors i, i and h may be operated for test purposes even when the thermostat switch or the low pressure switch is open.

A signal lamp |85, connected to the armature circuit of the fan motor m; indicates'whenthis motor is in operation, and a signal lamp |66, connected to the armature circuit of the compressor motor, indicates when the latter motor is in operation.

Prom the foregoing it will be seen that the motor m for circulating air past the rei'rigerating coil 3l must be in operation before' any oi the other motors can be started. This motor is started by pressing the start push button and may be kept running continuously while the car is in service. When the start push button is closed, the relays o and nbecome energized and the closure oi the relay o causes the operation ofthe starter l?. This starter makes up a circuit to the magnet of the run relay 1' and the latter may be energized by depression oi the "run" push button switch. The starter P and the run relay r, combined, make up a control circuit for thestarter S through the low pressure switch 33 and thermostat switch t and when these switches are closed this starter will cause the fan motor and pump motor associated with the condensing coil topperate. Il' the thermostat switch is open the latter motors will not operate; when the thermostat switch closes, due to rising temperature in the car, these motors win' be automatically started. Under some unusual conditions in the refrigerating systemf, the low pressure switch 'may open and if this occurs the control circuit .j will stop.

'Ihe starter S controls the starter V and hence the latter cannot operate to start the compressor motor h unless the motors i and j, which circulate air and water around the condenser coil, as well as the fan motor m, are in operation. 'I'he control circuit from the starter S to the starter V leads through the high pressure switch 32, and if this should open it will interrupt the circuit to the starter V and cause the stoppage of the compressor motor, but the motors i and 7' will continue in operation in order to cool the condenser coil as quickly as possible. When the high pressure switch again closes, the starter V will again operate to start the compressor, providing all oi' the other motors are in operation. When the starter V operates to start the compressor motor, a circuit to the relay a: is closed by the starter and this relay cuts out a section of the ileld resistance oi the generator to increase its field strength and compensate for the additional load placed on the generator.

To stop all of the motors it is merely necessary to depress the stop push button, which will cause the relay o to open the control circuit to the starter P, and since this starter, through the inter-locks described, controls the operation ot the various other motors, all of the motors will be prevented from operating.

It the voltage on the supply wires falls below a predetermined value, the relay n will open the circuit ot relay o and the latter will open the control circuit to the starter P and thereby prevent operation of any of the motors.

While I-have shown separate motors i and i for operating a fan and a pump, these motors are connected in parallel and are started and stopped simultaneously, and it will be evident that the fan and pump might be operated by a single motor if desired.

What I claim is:

l. In a, railway car, in combination, a storage an axle driven generator normally conbattery, nected for charging the same, a third brush excited ileld for said generator. a resistor in the circuit of said field, a plurality of motors adapted for operation from a main circuit supplied by the battery and generator, a starter for each motor, circuits interlocking said starters for operation only in sequence, and means associated with certain of said starters to automatically cut out sections of said resistor as the associated mo- 'Iii tor is started to provide additional current from the generator to compensatefor the additional load thereon.

2. In a railway car, in combination, a storage battery, an axle driven generator normally connected for charging the same, a third brush excited ileld for said generator, a resistor in the circuit of said eld, a plurality of motors adapted for operation from a main circuit supplied by the battery 'and generator, a starter for each motor, circuits interlocking said starters for operation only in sequence, manual means to initiate the operation of the first starter, a marginal relay in the circuit of said manual means, said relay being adjusted to remain closed to permit operation of the starter only i! the voltage of the main circuit remains suiliciently high to permit safe operation of the motors under'normal load, and means associated with certain of said starters to automatically out out sections of said resistor as the associated motor is started to provide additional current from the generator to compensate for the additional load thereon so as to maintain the voltage high enough to hold said relay closed.

3. In combination with af source of current subject to voltage fluctuations, a motor adapted to run on current from said source but requiring protection against running on low voltage, a starter relay combination to connect said motor to said source for safe starting, a manual button to initiate operation of said relay combination, a low voltage protection means interposed between said button and starter relay combination and including a relay whose contacts must be closed to supply current for operation of said starter relay combination, a marginal relay adapted to be closed or to stay closed only if the voltage applied to its holding coil is sumcient for the 'proper operation of said motor, a circuit including the coils of bothl relays, said button and said source, and contacts on each relay which when closed lock a holding circuit for the other relay whereby jarring which might vibrate the marginal relay contacts causes positive opening of the other relay so that current pulsations are not given to the starter relay combination.

4. A relay circuit arrangement adapted to supply current from a source of iiuctuating potential to the starter of a motor which must be protected against running on low voltage, including, in combination, a marginal relay whose holding coil functions only on potentials safe for the motor, and a` relay having contacts closed by itscoil to supply current from the source to operate said starter, means to manually complete a circuit through the coils of said relays and said source to close them, circuits including contacts thus closed whereby each relay completes a holding circuit for the other whereby a jarring ofthe marginal relay contacts suiilciently to open them causes positive opening of the other relay contacts to prevent pulsating current reaching the said starter.

5. In a railway car having a main electrical circuit supplied by a battery and an axle driven charging generator, in combination, a motor requiring protection against running on low voitage, a starter for said motor, means to supply current tov said starter from said source for starting and running said motor only when the voltage is suiiiciently high including a relay having contacts closed by its coil to close the supply circuit to said starter, a marginal relay whose holding coil functions only on potentials safe for the motor, means to momentarily close a circuit manually to energize the coils of both said relays, circuits including contacts thus closed on each relay to close a holding circuit for the other relay, the ilrst mentioned relay having its coil arranged to hold it closed on less than the desired running voltage of said motor for the purpose described.

6. In combination, a supply circuit, an electric motor,.a circuit for the motor, a contactor for closing the motor circuit, a control circuit for the contactor, a normally open relay for closing said control circuit, a low voltage relay adapted to release its armature and open the magnet circuit of said first mentioned relay at a predetermined low voltage in the supply circuit, and said first mentioned relay constructed to maintain said control circuit closed at alower voltage than that at which said low voltage relay releases its armature and adapted to open said control circuit only when its magnet circuit is opened by said low voltage relay.

7. In combination, a supply circuit, an electric motor, a circuit for the motor, a contactor for closing the motor circuit, a control circuit for f the contactor, a normally open relay for closing said control circuit, a low voltage relay adapted to open the magnet circuit of said ilrst mentioned relay at a predetermined low voltage in the supply circuit, and said first mentioned relay adapted 'to maintain said control circuit closed until the magnet circuit o! said first mentioned relay is opened by the second mentioned relay, a circuit for initially energizing the coils of said relays, a manually operable switch for closing said latter circuit, and holding circuits for said magnets, each relay, when energized, adapted to close the holding circuit of the other relay.

8. In combination, a supply circuit, an electric motor, a circuit for the motor, a contactor for closing the motor circuit, a control circuit for the contactor, a normally open relay for closing said control circuit, a low voltage relay adapted to close the magnet circuit of said iirst mentioned relay when the-voltage in the supply circuit is above a predetermined value and to release its armature and open said magnet circuit when the voltage falls to said value, and said rst mentioned relay constructed to maintain said control circuit closed at a lower voltage than that at which said low voltage relay releases its armature and adapted to open said control circuit only when its magnet circuit is opened by said low voltage relay.

9. In combination, a supply circuit, an electric motor, a circuit for the motor, a contactor for Aclosing the motor circuit, a control circuit for the contactor, a normally open relay for closing said control circuit, a low voltage relay adapted to close the magnet circuit of said first mentioned relay when the voltage in the supply circuit is above a predetermined value and to open said magnet circuit when the voltage falls to said value, and said ilrst mentioned relay, when energized, adapted to maintain said control circuit closed until its magnet is de-energized by the opening of the low voltage relay, a circuit f for initially energizing the coils of said relays,

` a manually operable switch for closing said latter circuit, and holding circuits for said magnets,

each relay, when energized, adapted to close the holding circuit of the other relay. y

JOSEPH F. 

